1. Field of the Invention
The present invention relates to an ultrasonic motor for driving a member to be driven by generating an ellipsoidal vibration on an impressed vibrator, and to a lens apparatus using the ultrasonic motor.
2. Description of the Related Art
An ultrasonic motor has been conventionally used as a driving source for driving, for example, a lens mechanism or a camera, taking advantage of silent operation, driving capability from low speed to high speed, and high torque output. For example, an ultrasonic motor disclosed in Japanese Patent No. 4,652,784 includes an annular member to be driven having a rotation axis, and multiple vibrators. Each vibrator is in a so-called pressurized contact condition with respect to the member to be driven, i.e., the vibrator is in contact with the member to be driven in a state of being impressed against the member to be driven. The vibrators are arranged on the annular member to be driven at predetermined intervals. When an ultrasonic vibration is excited on the vibrator under the pressurized contact condition, an ellipsoidal motion is generated on the vibrator at a portion in contact with the member to be driven, so that the member to be driven is driven to rotate about the rotation axis of the member to be driven. The pressurized contact condition of the vibrator with respect to the member to be driven is obtained by biasing, with use of a plate spring, a portion of the vibrator corresponding to a vibration node set near the center of the vibrator. An impressing force of the plate spring is adjusted by a screw and an adjustment washer provided near a fixing portion of the plate spring.
However, the ultrasonic motor disclosed in Japanese Patent No. 4,652,784 includes a complicated adjustment mechanism for adjusting the impressing force applied to the vibrator with a number of components constituting the adjustment mechanism. In addition, the plate spring of the adjustment mechanism is formed into a linear form that can stay within the annular form, and therefore, the plate spring has a small overall length and a high spring constant. As a result, slight adjustment of the plate spring leads to a significant change in the impressing force, and hence fine adjustment is needed.